Electron excitation effects on NMR shieldings in small organic molecules

As an initial examination of the effects of electron excitation on chemical shieldings of protons, carbon-13 and oxygen-17 nuclei, we report values obtained from ab initio calculations invoking a substituted orbital description of the excited states. For the series of molecules studied, excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital gives shifts from −10 to 31 ppm for carbon-13 but less than 1 ppm for hydrogen centers. In contrast, excitation from an oxygen n (2p) orbital in formaldehyde has an effect on oxygen shielding of several hundred ppm.